1. Field of the Invention
The present invention relates to a tuning-fork mechanical resonator for a mechanical clock movement with free escapement, comprising an oscillator of the tuning fork type, of which at least one first oscillating prong is intended to oscillate about a first axis and bears at least one first pin associated with at least one first fork tooth of a pallet assembly to cause this assembly to pivot between first and second angular positions and alternately lock and release an escapement wheel.
In the known way, such a mechanism makes it possible, in conjunction with a source of mechanical energy, to sustain the oscillations of the oscillator that is the tuning fork and thus define a resonator.
The high quality factor of an oscillator such as a tuning fork, namely around ten to fifty times as high as that of a conventional balance spring oscillator, makes it attractive for horology applications.
Moreover, the present invention also relates to a clock movement fitted with such a resonator and to a timepiece, particularly although not exclusively of the wristwatch type, fitted with such a clock movement.
2. Description of Related Art
Many horology devices comprising a tuning fork by way of an oscillator have already been disclosed in the prior art.
By way of example, patent FR 73414 A, granted in the name of Louis-François-Clément Breguet on the basis of an application filed in 1866, describes a pendulum clock the mechanical oscillator of which is a tuning fork. A first prong of this tuning fork bears a pin so as to be constrained to move within a hole provided in a pallet assembly having two arms designed to collaborate with an escapement wheel, in order alternately to lock and release the latter, the pallet assembly being mounted on a frame component of the clock movement so as to pivot. The escapement thus designed is not of the free escapement type because, on the one hand, the pallet assembly is in permanent contact with the escapement wheel and, on the other hand, the pin fixes the pallet assembly to the prong of the tuning fork and therefore never leaves the pallet assembly. Such an escapement therefore has the corresponding disadvantages, namely in particular wear and chronometric disturbance both of which are greater than with a free escapement.
As far as wristwatches are concerned more particularly, Max Hetzel has been behind a great many patented inventions relating to the use of a tuning fork as an oscillator, which have led to the production of the Accutron (registered trade name) wristwatch marketed by the company Bulova Swiss SA.
The Accutron watch however comprises an electronic resonator given that each prong of the corresponding tuning fork bears a permanent magnet associated with an electromagnet mounted fixedly on the frame of the watch. The operation of each electromagnet is slaved to the vibrations of the tuning fork, via the magnets it carries, such that the vibrations of the tuning fork are sustained by the transmission of periodic magnetic impulses from the electromagnets to the permanent magnets. One of the prongs of the tuning fork operates a pawl that allows the wheels of the watch gear train to be turned. This construction does not lend itself to the use of the pawl for sustaining the oscillations of the tuning fork.
U.S. Pat. No. 2,971,323 for example, derived from a filing dating from 1957, describes such a mechanism which, however, cannot be used for creating a purely mechanical watch, i.e. a watch that has no electronic circuits. Indeed, in market terms, there is a real need for purely mechanical timepieces that run with greater precision than the known timepieces.
It should be pointed out that the Accutron timepiece is still currently marketed by the company Bulova Swiss SA.
Patent CH 594201, derived from a filing dating from 1972, describes a double-oscillator resonator system. The frequency stability of the oscillations of a tuning fork is put to use, through magnetic interaction, to stabilize the oscillations of a balance of conventional form, which therefore has a lower quality factor than the tuning fork. To this end, the prongs of the tuning fork, on the one hand, and the balance on the other hand, bear permanent magnets designed to collaborate with one another. The corresponding interaction makes it possible both to sustain the oscillations of the tuning fork and to stabilize the oscillations of the balance in terms of frequency.
However, although that is not explicitly evident in that patent, it is obvious that this mechanism has to be coupled to a mechanical escapement in order to convert the periodic oscillations of the balance into a one-way movement capable of driving the wheels of a gear train. Thus, it is probable that the balance is coupled to a conventional mechanical escapement designed to sustain the oscillations thereof. As a result, the mechanism described in that document allows an improvement in the frequency stability of the oscillations of a balance, but does so at the expense of a complexity and a bulkiness that are far higher than those of a conventional mechanism with just one oscillator. Further, the high quality factor of the tuning fork is only partially put to use in the solution presented because, in the end analysis, it is the balance that controls the movements of the gear train, in a similar way to the operation of conventional systems.
Alternative solutions better suited to the spatial constraints specific to the construction of a wristwatch, have also been disclosed. Specifically, U.S. Pat. No. 3,208,287, derived from a filing dating from 1962, describes a resonator comprising a tuning fork coupled to an escapement wheel via magnetic interactions. More specifically, the tuning fork bears permanent magnets collaborating with the escapement wheel, the latter being made from a magnetically conducting material. The escapement wheel is kinematically connected to a source of energy which may be mechanical or take the form of a motor, whereas it has openings, in its thickness, such that it forms a magnetic circuit of variable reluctance when driven in rotation, in relation to the magnets borne by the tuning fork.
Therefore, permanent interaction of substantial intensity occurs, between the tuning fork and the escapement wheel, that can be qualified as magnetic locking, such a construction therefore consisting of a non-free escapement. The supply of energy from the escapement wheel to the tuning fork in order to sustain the oscillations thereof, even though small, occurs continuously and constitutes a source of disruption that is not insignificant in terms of the isochronism of these oscillations. Likewise, the escapement wheel is continuously guided by the tuning fork.
Thus, the type of interaction used in this construction is similar to a contact, and this is detrimental from a precision operation standpoint.
Leaving the Louis-Francois-Clement Breguet pendulum clock to one side, all of these mechanisms use a magnetic interaction and none lends itself to the creation of a purely mechanical timepiece, namely one that contains neither electronics nor magnetic interaction.